Shaped-beam antenna for earth coverage from a stabilized satellite

ABSTRACT

The apparatus of the present invention provides an antenna having a beam shaped for optimum earth coverage from a synchronous satellite. Due to the difference in range and atmospheric attenuation from a synchronous satellite to various points on earth, a conventional beam with maximum gain toward the center of the earth, is inefficient because it has the highest gain where the least gain is required. Since the paths tangential to the earth are longest and since they traverse through more atmosphere, the gain of the disclosed antenna is highest in this region and decreases to a minimum for the path normal to the earth. In addition, the beam pattern of the antenna has &#39;&#39;&#39;&#39;flat portions&#39;&#39;&#39;&#39; at the edge to allow for stabilization errors of the satellite whereby equal effective signal is provided over the entire portion of the earth covered by the antenna beam. The antenna generates a beam pattern that is rotationally symmetrical and has the capability of dual orthogonal polarization.

1 1 amazes Pn'mary ExaminerEli Lieberman AttomeysJames K. Haskell andRobert H. Himes ABSTRACT: The apparatus of the present inventionprovides an antenna having a beam shaped for optimum earth coverage froma synchronous satellite. Due to the difference in range and atmosphericattenuation from a synchronous satellite to various points on earth, aconventional beam with maximum gain toward the center of the earth, isinefficient because it has the highest gain where the least gain isrequired. Since the paths tangential to the earth are longest and sincethey traverse through more atmosphere, the gain of the disclosed antennais highest in this region and decreases to a minimum for the path normalto the earth. In addition, the beam pattern of the antenna has flatportions" at the edge to allow for stabilization errors of the satellitewhereby equal effective signal is provided over the entire portion ofthe earth covered by the antenna beam. The antenna generates a beampattern that is rotationally symmetrical and has the capability of dualorthogonal polarization.

SHAPED-BEAM ANTENNA FOR EARTH COVERAGE FROM A STABILIZED SATELLITEBACKGROUND OF THE INVENTION Contemporary antennas are simple ormultimode horns and planar arrays of nominally half-wave elements spacedof the order of half to three-quarters of a wavelength. Simple ormultimode horns do not give the proper shaping or are of extremelynarrow bandwidth. An array, on the other hand, is extremely complexbecause of the large number of elements, is quite narrow band and isquite lossy because of complex feed network. Further, if polarizationdiversity is desired, the complexity is more than doubled.

SUMMARY OF THE INVENTION It is well known that a Lambda function,.I,(u)/u, aperture distribution will give a rotationally symmetricalsector-shaped pattern exactly but would require an infinite aperture. Inaccordance with the present invention, this aperture distribution isapproximated by a horn array wherein a larger center horn provides adistribution approximating the main lobe of the J,(u)/Au function whilea ring of smaller horns 180 out of phase with the center hornapproximates the distribution of the first minor lobe of the J, (u)/ufunction. The resulting horn array generates a concave shaped beam withalmost perfect rotational symmetry and with minimum absolute gaingreater than 18 db. 9.5 from center. When used on a stabilized satelliteat synchronous altitude, 95 from center is at the earths edge where oneusually experiences the weakest signal. In addition to its use onsatellites, the array of the present invention is also useful as thefeed for a Cassegrain antenna.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates the nine-horn arrayof the invention, in

perspective, without feed system;

FIG. 2 shows a cross section of the nine-horn array of FIG.

FIG. 3 shows a schematic diagram of a feed system for the nine-hornarray of FIG. 1;

FIG. 4 illustrates measured patterns of the nine-horn array of FIG. 1without multimoding;

FIG. 5 illustrates measured patterns of the nine-horn array of FIG. 1with center horn multimoded; and

FIG. 6 shows the ideal pattern for earth coverage.

DESCRIPTION Referring now to FIGS. 1 and 2 of the drawings, the array ofthe present invention includes a large conical center horn 10 which ismounted to a disk 12 by means of a standoff support 14 attached to aflange 15 at the neck portion thereof. In accordance with the invention,an integral number of smaller conical horns 16-23 are disposed inalignment with and at equal intervals about the large conical centerhorn 10. Although a number of horns equal to an exponential of the basetwo can be used, i.e., 2, 4, 8, l6 it has been found that the use of theeight conical horns l623 is advantageous from the standpoint of relativeaperture area and simplicity of the driving apparatus. In order for thesmaller conical horns 16-23 to surround the center horn 10 withoutleaving a gap, the diameter of the respective apertures thereof are madeequal to 0.618 times the diameter of the aperture of the center horn 10.Under these circumstances, the ratio of the area of the apertures of theconical horns 16-23 to the area of the aperture of the center horn 10 is3. In addition, the flare angle of the center horn 10 and peripheralhorns 16-23 control the phase front over the respective horn aperture.Larger flare angles produce more convex phase fronts and smaller flareangles less convex phase fronts. In this respect, the peripheral horns16-23 can be tilted in with respect to the center horn 10 to achieveadditional control of the phase over the entire aperture of the array.The disk 12 includes eight equally spaced radial slots adapted toaccommodate the neck portions of the conical horns 16-23 therebyenabling the respective flanges thereof to be attached thereto. Thecenter horn 10 may be multimoded in accordance with known techniques oras described in copending application for patent titled BroadbandMultimode Horn Antenna by James S. Ajioka, Ser. No. 771,178 filed Oct.28, 1968 and assigned to the same patentee as is the present case. Thespecification of this patent is incorporated herein by reference.

Referring to FIG. 3 there is shown a schematic diagram of an apparatusfor feeding the nine-horn array of FIG. 1, As previously explained, thenine-horn array of the present invention approximates the main lobe 27and first minor lobe" 28 of a Lambda function .I,(u)/u as indicated bythe characteristic 30, FIG. 3. To achieve this, an input 25 passesthrough a power divider 26 and connects to the input flange 15 of thecenter horn 10. The antenna pattern is shaped by the division of powerbetween the center horn l0 and the surrounding smaller horns l623. Toachieve the division to approximate the characteristic 30, the powerdivider 26 splits the power in a manner to direct percent of the inputpower to the center horn 10. The remaining output from power divider 26is connected to the shunt arm of a magic tee 32, the series arm of whichis terminated. Output arms from the magic tee 32 are, in turn, connectedto shunt arms of magic tees 33, 34, the series arms of which are againterminated. The output arms of magic tees 33, 34 are then connected tothe shunt input arms of magic tees 35, 36, 37, 38, the series arms ofwhich are terminated. Finally, the output arms of the magic tees 35 36,37, 38 are connected to the input flanges of the smaller conical horns16-23. The inputs of the conical horns 16-23 are oriented in a mannersuch that the polarity of the signal ap pearing at the respectiveapertures thereof are out of phase from the signal appearing at theaperture of the large conical horn 10. The 180 phase difference isemployed because of the opposite polarity of the first minor lobes 28relative to the main lobe 29 of the characteristic 30. All of the inputsto the conical horn l0 and to the conical horns 16-23 are designed tolaunch a dominant TE mode which may be multimoded in the case ofhorn 10.In the event that dual orthogonal polarization is desired, an orthogonalmode transducer (not shown) is interposed between each horn 10, 16-23and the feed network of FIG. 3 and a similar network connected from theorthogonal mode transducers used for the orthogonal mode. Also it isunderstood that the terminations can be removed from any or all of themagic tees 32-38 for the purpose of providing antenna-pointing errorcorrection information commonly known as monopulse operation. In thecase of operation from a stabilized satellite, error correction is notrequired as there is no movement between the transmitter and receiverother than minor variations resulting from the stabilization.

Referring to FIG. 6 there is shown an ideal pattern 40 for earthcoverage versus a conventional pattern 41 for operation fromsynchronized satellites 42, 43, respectively. The ideal pattern 40 hasshoulders 44, 45 which are 3.6 db. up from the center beam intensitywhereby maximum signal is directed toward the edge of the earth as seenfrom the satellite 42 thereby providing a substantially uniform signalover the portion of the earths surface covered. The shoulders 44, 45 ofpattern 40 are of the order of one degree in width to allow for minorerrors in the orientation of the satellite 42. As contrasted with theideal pattern 40, the conventional pattern 41 is the weakest at the edgeof the earth where maximum signal is needed. Also, a substantial portionof the pattern 41 is wasted as the energy therein never falls on theearth.

In the operation of the nine-horn array of FIG. 1, the ideal pattern 40is approximated by using a flare angle of the order of 10 for the centerhorn l0 and for the peripheral horns 17-23 and by adjusting the powerdivider 26 to deliver 95 percent of the input power to the center horn10 whereby the remaining 5 percent of the input power is divided betweenthe surrounding horns 16-23 by the magic tees 32-38. A domi- 1proximates that ofvthe ideal pattern 40, FIG. 6. The

nominally 180 nant TE mode is launched in each of the horns 16-23 and inthe center horn 10. Under these circumstances, the nine-horn array ofFIG. 1 generates a beam having the H-plane pattern 50, the E-planepattern 52 and the diagonal plane pattern 54 shown in FIG. 4. As shownin the drawing, the patterns 50, 52, 54, in addition to havingrotational symmetry and polarization purity, have ashoulder-to-shoulder.width of 19 which apside lobes in the patterns 50,52, 54 may be minimized in the manner described in the aforementionedapplication. titled, -Broadband Multimode Horn Antenna by multimodingthe center horn 10. With the center horn l multimoded in this manner,the nine-horn array of FIG. 1 generates a beam having the H-planepattern 56, the E-plane pattern 58 and the diagonal plane pattern 60shown in FIG. wherein the size of the side lobes apparent in ,thecorresponding patterns 50, 52, 54 are substantially reduced. As before,the shoulder width of the patterns 56, 58, 60, are each 19 whichapproximates that of the ideal pattern 40 for earth coverage from asynchronous satellite. in other applications such as the feed for aCassegrain antenna, other power splits by the power divider 26 may berequired and larger flare angles used depending on size of reflector andfrequency.

What is claimed is:

1. An antenna system comprising a conductive center horn ofpredetermined size, said center horn having an input at one extremityand an aperture at the remaining extremity thereof; a plurality of'nolessthan four and an integral power of two peripheral'horns, eachhavingan input at one extremity and an aperture at the remaining extremitythereof, each being symmetrical abouta longitudinal axis andeachbeingofa uniform size smaller than said predetermined size, said plurality ofI horns being disposed at uniform intervals about and at the same pointalong said center horn and aligned in the same direction as said centerhorn; means coupled to said input at said one extremity of said centerhorn for launching a first signal of predetermined frequency, phase, andpower from said center horn; and means coupled to said respective inputsof said plurality of horns for simultaneously launching a second signaltherefrom, said secondsignal having a frequency equal to saidpredetermined frequency, a power that is only a fraction of saidpredetermined power and a phasethat is relative to said predeterminedphase.

2. The antenna system as defined in claim 1 wherein the total area ofsaid apertures of said plurality of horns is substantially three timesthe area ofsaid aperture ofsaid center horn.

3. The antenna system as defined in claim 1 wherein said aperture ofsaid center horn and said apertures of said plurality of horns are in acommon plane.

4. The antenna system as defined in claim 1 additionally including meansdisposed in said center horn for multimoding said center horn.

5. An antenna system for generating a predetermined antenna pattern inresponse to an applied signal, said system comprising a conductiveconical center horn of predetermined size, said conical center hornhaving an input at one extremity and an aperture at the remainingextremity thereof; eight conductive conical peripheral horns, eachhaving an input atone extremity and an aperture at the remainingextremity thereof and each being of a uniformsize smaller than saidpredetermined size, said eight peripheral horns being disposed atuniform intervals about said center horn; and means coupled to saidinput of said conical center horn and to said respective inputs of saideight conical peripheral horns and responsive to said applied signal forlaunching a major portion of the power ofsaid signal from said conicalcenterhorn asawave ina TE dominant mode with a predetermined phase andfor launching the remaining portion of the power of said signal equallyfrom said eight peripheral horns as respective waves in TE dominantmodes with a phase nominally 180 relative to said predetermined phase.

6. The antenna systemas defined in claim 5 wherein the respectivediameters of said eight peripheral conical horns equal 0.618 times thedlametero said aperture ofsaid conical center horn.

7. The antenna system as defined in claim 5 additionally including meansin said conical center horn for multimoding said conical center horn.

8. An antenna system for generating a predetermined antenna pattern inresponse to an applied signal, said system comprising a conductiveconical center horn of predetermined size, said conical center hornhaving an input atone extremity tees having a shunt input arm and firstand second output arms, said first and second output arms of said first,second,

third and fourth magic tees being connected, respectively,to

said inputs'of said eight conical peripheral horns to launch signals ofpredetermined polarity therein, said first and second output arms ofsaid fifth and sixth magic tees being connected to said shunt armsofsaid first, second, third, and fourth magic tees, and said first andsecond output arms of said seventh magic tee connected to said shuntarms of said fifth and sixth magic tees; and means including a powerdivider responsive to said signal and having outputs connected to saidinput of said conical center horn and said shunt arm of said seventhmagic tee for directing a major portion of the power of said signal tosaid center horn and the remaining portion of the power of said signalto said shunt arm of said seventh magic tee.

9. The antenna system as defined in claim 8 wherein said power dividerdirects percent of the power of said signal to said center horn and theremaining 5 percent thereof to said shunt arm of said seventh magic tee.

1. An antenna system comprising a conductive center horn ofpredetermined size, said center horn having an input at one extremityand an aperture at the remaining extremity thereof; a plurality of noless than four and an integral power of two peripheral horns, eachhaving an input at one extremity and an aperture at the remainingextremity thereof, each being symmetrical about a longitudinal axis andeach being of a uniform size smaller than said predetermined size, saidplurality of horns being disposed at uniform intervals about and at thesame point along said center horn and aligned in the same direction assaid center horn; means coupled to said input at said one extremity ofsaid cenTer horn for launching a first signal of predeterminedfrequency, phase, and power from said center horn; and means coupled tosaid respective inputs of said plurality of horns for simultaneouslylaunching a second signal therefrom, said second signal having afrequency equal to said predetermined frequency, a power that is only afraction of said predetermined power and a phase that is nominally 180*relative to said predetermined phase.
 2. The antenna system as definedin claim 1 wherein the total area of said apertures of said plurality ofhorns is substantially three times the area of said aperture of saidcenter horn.
 3. The antenna system as defined in claim 1 wherein saidaperture of said center horn and said apertures of said plurality ofhorns are in a common plane.
 4. The antenna system as defined in claim 1additionally including means disposed in said center horn formultimoding said center horn.
 5. An antenna system for generating apredetermined antenna pattern in response to an applied signal, saidsystem comprising a conductive conical center horn of predeterminedsize, said conical center horn having an input at one extremity and anaperture at the remaining extremity thereof; eight conductive conicalperipheral horns, each having an input at one extremity and an apertureat the remaining extremity thereof and each being of a uniform sizesmaller than said predetermined size, said eight peripheral horns beingdisposed at uniform intervals about said center horn; and means coupledto said input of said conical center horn and to said respective inputsof said eight conical peripheral horns and responsive to said appliedsignal for launching a major portion of the power of said signal fromsaid conical center horn as a wave in a TE11 dominant mode with apredetermined phase and for launching the remaining portion of the powerof said signal equally from said eight peripheral horns as respectivewaves in TE11 dominant modes with a phase nominally 180* relative tosaid predetermined phase.
 6. The antenna system as defined in claim 5wherein the respective diameters of said eight peripheral conical hornsequal 0.618 times the diameter of said aperture of said conical centerhorn.
 7. The antenna system as defined in claim 5 additionally includingmeans in said conical center horn for multimoding said conical centerhorn.
 8. An antenna system for generating a predetermined antennapattern in response to an applied signal, said system comprising aconductive conical center horn of predetermined size, said conicalcenter horn having an input at one extremity and an aperture at theremaining extremity thereof; eight conductive conical peripheral horns,each having an input at one extremity and an aperture at the remainingextremity thereof and each being of a uniform size smaller than saidpredetermined size, said eight peripheral horns being aligned in thesame direction as said center horn and disposed at uniform intervalsthereabout with the apertures thereof in a plane common with saidaperture of said center horn; first, second, third, fourth, fifth,sixth, and seventh magic tees, each of said magic tees having a shuntinput arm and first and second output arms, said first and second outputarms of said first, second, third and fourth magic tees being connected,respectively, to said inputs of said eight conical peripheral horns tolaunch signals of predetermined polarity therein, said first and secondoutput arms of said fifth and sixth magic tees being connected to saidshunt arms of said first, second, third, and fourth magic tees, and saidfirst and second output arms of said seventh magic tee connected to saidshunt arms of said fifth and sixth magic tees; and means including apower divider responsive to said signal and having outputs connected tosaid input of said conical center horn and said shunt arm of saidseventh magic tee for directing a major portion of the power of saidsignal to saId center horn and the remaining portion of the power ofsaid signal to said shunt arm of said seventh magic tee.
 9. The antennasystem as defined in claim 8 wherein said power divider directs 95percent of the power of said signal to said center horn and theremaining 5 percent thereof to said shunt arm of said seventh magic tee.